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MODIFY_LDT(2)              Linux Programmer's Manual             MODIFY_LDT(2)



NAME
       modify_ldt - get or set ldt

SYNOPSIS
       #include <sys/types.h>

       int modify_ldt(int func, void *ptr, unsigned long bytecount);

DESCRIPTION
       modify_ldt()  reads  or writes the local descriptor table (ldt) for a process.  The ldt is a per-process memory
       management table used by the i386 processor.  For more information on this table, see an  Intel  386  processor
       handbook.

       When  func is 0, modify_ldt() reads the ldt into the memory pointed to by ptr.  The number of bytes read is the
       smaller of bytecount and the actual size of the ldt.

       When func is 1, modify_ldt() modifies one ldt entry.  ptr points to a user_desc structure  and  bytecount  must
       equal the size of this structure.

       The user_desc structure is defined in <asm/ldt.h> as:

           struct user_desc {
               unsigned int  entry_number;
               unsigned long base_addr;
               unsigned int  limit;
               unsigned int  seg_32bit:1;
               unsigned int  contents:2;
               unsigned int  read_exec_only:1;
               unsigned int  limit_in_pages:1;
               unsigned int  seg_not_present:1;
               unsigned int  useable:1;
           };

       In Linux 2.4 and earlier, this structure was named modify_ldt_ldt_s.

RETURN VALUE
       On  success,  modify_ldt() returns either the actual number of bytes read (for reading) or 0 (for writing).  On
       failure, modify_ldt() returns -1 and sets errno to indicate the error.

ERRORS
       EFAULT ptr points outside the address space.

       EINVAL ptr is 0, or func is 1 and bytecount is not equal to the size of the structure modify_ldt_ldt_s, or func
              is 1 and the new ldt entry has invalid values.

       ENOSYS func is neither 0 nor 1.

CONFORMING TO
       This call is Linux-specific and should not be used in programs intended to be portable.

NOTES
       Glibc does not provide a wrapper for this system call; call it using syscall(2).

SEE ALSO
       vm86(2)

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2007-06-01                     MODIFY_LDT(2)